This invention relates to vacuum drying of articles and, in particular, to apparatus and method for rapidly removing large amounts of water without sublimation.
A wide variety of products today include small components which must be completely clean of foreign material in order for the products to operate reliably. Some examples include the internal components of anti-lock braking systems, electrical connectors, metering valves for fuel injectors, and metal lids and ceramic packages for integrated circuits. The cleaning of such components typically includes several rinse operations followed by immersion in a tank containing a dense chlorofluorocarbon (CFC) compound, such as Freon 113 (C.sub.2 ClF.sub.3). Since the CFC is chosen to be more dense than water, the water is displaced from the component and floats to the surface of the CFC where it is skimmed or drawn off. The component is removed from the tank and the CFC evaporates rapidly. Because of the need to eliminate CFCs from the atmosphere, many companies are seeking alternatives to drying with CFCS.
Drying articles in a partial vacuum is known in the art. Water "boils" at a lower temperature when the ambient pressure is reduced. Since the change from the liquid phase to the gas or vapor phase requires heat (540 calories per gram or 4.2.times.10.sup.3 J/kg), the evaporating water absorbs heat from the remaining water or from the article being dried, cooling the remaining water or the article. If there is a large amount of water, i.e. more water to evaporate than can be kept above 0.degree. C. by the heat stored in the water and in the article, then the water freezes at some point in the drying cycle and thereafter sublimes. Sublimation is a far slower process than evaporation because the vapor pressure of the water is so low, about four tort at 0.degree. C. One could lower the pressure in the chamber even further, but the rate of sublimation does not increase significantly.
It is known in the semiconductor art to pass previously dried silicon semiconductor wafers through an evacuated oven to remove trace amounts of water from the lids and packages prior to sealing the lids to the packages. It is recognized in the prior art that evaporation from the liquid phase is more rapid than evaporation from the solid phase. For example, U.S. Pat. No. 4,468,866 --Kendall--discloses a system for drying new pipelines for natural gas by evacuating the pipeline at a controlled rate while monitoring the temperatures in and above a sample of water. An evacuation rate is chosen which prevents the formation of ice.
These techniques do not address the need for removing large amounts of water, as defined above, from articles after rinsing. For example, the lids for integrated circuit packages are typically gold plated Kovar squares, about 0.75 inches on a side. The stamping and plating operations for making the lids leave residues of contaminants which are removed by successive rinses in deionized water, perhaps including a surfactant. Within a batch of lids, many pairs of lids may be joined together by a thin layer of water. If the lids are dried using prior art techniques, the result is a mass of frozen lids surrounded by some dry lids. The mass may dry eventually but the time for processing the batch is prohibitive, e.g. 8-10 hours. Even so, the lids will be cold and water vapor in the air may condense on the surfaces of the lids as soon as the lids are removed from the dryer.
Other articles present different problems. A very difficult surface to dry is the inside of a cylinder having one closed end. Considering the ratio of length to diameter as the aspect ratio, a high aspect ratio permits the water to remain trapped in a cylinder. One example of such an article is a female electrical connector having a plurality of such cylinders in an array. The small diameter of the cylinders, e.g. 0.06 inches or less, permits water in the cylinders to be retained by surface tension. If the water were permitted to freeze, the cylinders could rupture.
In the prior art, it is known to dry articles in a hot-wall vacuum chamber. The walls are heated to prevent condensation of the water removed from the articles. This system does not prevent sublimation, as shown by an experiment in which a shallow stainless steel pan is filled with approximately one cm. of water at 20.degree. C. The hot-wall vacuum chamber is heated to 135.degree. C. and the pan is placed on the heated bottom of the chamber. The chamber is closed and the pressure within the chamber is reduced. Initially the water boils but, as it boils, the heat removed by vaporization cools the remaining water and the boiling slows. The water is cooled despite the heat being conducted through the pan from the bottom of the chamber. After about seven minutes, a layer of ice forms in the pan. If the pressure in the chamber is restored to atmospheric and the pan is removed at this time, the pan is hot and has a mass of ice in the middle off it. Thus, it is difficult to prevent sublimation in vacuum drying apparatus.
In view of the foregoing, it is therefore an object of the invention to dry articles in a vacuum by evaporating water only from the liquid phase.
Another object of the invention is to remove large amounts of water rapidly from articles in a vacuum drying apparatus without forming ice.
A further object of the invention is to provide a process for radiantly heating articles in a vacuum to prevent sublimation.
Another object of the invention is to provide an apparatus and method for drying articles having complex surfaces in which water can be trapped.
A further object of the invention is to provide a method and apparatus for drying articles in a vacuum without significantly changing the temperature of the articles.
Another object of the invention is to prevent ice formation in vacuum drying apparatus by controlling the rate of evaporation of the water.
A further object of the invention is to clean an article by subjecting the article to vacuum drying.